Galvanized coatings are commonly applied for corrosion protection of metal parts, and especially steel or iron parts. In the galvanizing process, a solidified layer of zinc is formed on a part's surfaces by immersing the part into molten zinc which contains approximately 1% lead. The main benefit of lead's presence in zinc is that it makes the process of galvanizing less demanding and less sensitive to many unfavorable circumstances, like insufficiently pickled, cleaned and even rusty steel surfaces, the absence of preheating and even drying when wet parts are immersed in molten zinc, and so on.
Unfortunately, lead cannot now be used for galvanizing steel parts coming into contact with drinking water, since small but significant quantities of lead from the galvanized parts dissolve in the water. Such dissolved lead may be accumulated in human and animal bodies with very deleterious results. Consequently, a number of state legislatures have passed laws which, since 1995, forbid the presence of lead in galvanized products which contact drinking water.
Attempts have been made to reduce the lead level from conventional levels, for example, containing a maximum of about 1.4% by weight lead in Prime Western grade zinc to a High Grade Standard requirement level of a maximum of about 0.03% by weight lead, or to a Special High Grade Standard requirement of a maximum of about 0.003% by weight lead (ASTM B6-87 standard). However, attempts to reduce lead to less than about 0.5% by weight of lead have resulted in insufficient wetting of steel parts and finished products having a high percentage of uncoated surfaces, that is, black or bare spots.
Other techniques have been developed in the galvanization process in conjunction with the use of lead to produce high quality galvanized coatings. Metal parts are usually treated with aqueous solutions prior to immersing the parts into the galvanizing bath. Metal parts typically undergo alkaline cleaning, rinsing, acidic pickling, and rinsing treatments. After the parts have been cleaned, rinsed, pickled and rinsed, they are usually immersed in a pre-flux solution which is an aqueous solution containing ammonium chloride or a mixture of ammonium chloride and zinc chloride. The application of pre-flux has been found to remove metal oxides, for example, iron oxide from steel part surfaces, to promote good wetting with the molten zinc.
A top flux is often employed in conventional processes. Top fluxes are typically comprised of the same ingredients as a pre-flux, except that the salts such as zinc chloride and ammonium chloride are molten in form and float on top of the galvanizing bath. Top fluxes have the further advantage that they reduce or eliminate metal spattering when steel articles are immersed into the galvanizing bath, which can occur if the article is still wet with aqueous solutions such as pre-flux.
As a metal part having a surface temperature substantially lower than the molten zinc is immersed in the molten zinc bath, a frozen layer of zinc is formed on the surface of the part. The frozen zinc layer forms before the molten zinc is able to adequately wet the surface of the steel, and hence there is poor adhesion of the frozen zinc onto the metal part surface. However, soon thereafter, the frozen layer on the part surface gradually melts. When the layer is totally melted, the metal surface is wetted by molten zinc and growth of a zinc-metal intermetallic alloy forms an integrated surface between the zinc and the metal part.
Perhaps the most significant benefit of lead is that it accelerates the melting of the frozen zinc layer on the surface of the part, and galvanization takes place more rapidly. This phenomenon is described in U.S. Pat. No. 5,437,738. Lead also significantly reduces surface tension of molten zinc and increases its fluidity. This results in better wetting of the steel surface to be coated and higher drainage after withdrawal of the parts from the kettle. Another technique that has been used to accelerate the rate of galvanization, is to preheat the parts after the parts are immersed in pre-flux. Preheating is typically conducted at high temperatures, for example, greater than 200.degree. C., so that the steel surfaces are wetted by molten zinc as rapidly as possible. Also, parts that are arranged in a batch have many contact surfaces with adjacent parts and require additional heat for drying. Since conventional pre-fluxes are decomposed or burned when heated, for example, to 200.degree. C. to 250.degree. C. for 3 to 15 minutes, a non-conventional, heat-stable pre-flux is needed.
It is desirable to provide a new galvanizing process that is low-lead or no lead for producing uniform, void-free coatings on metal parts. It is desirable to provide a galvanization process that essentially thoroughly dries the parts after the parts are immersed in a conventional pre-flux solution. It is also desirable to provide a galvanization process in which the top-flux treatment can be eliminated. It is desirable that the galvanizing process can be carried out in batch operation. In addition, it is desirable to provide a low-lead or no-lead galvanizing process with galvanizing baths that may contain low concentrations of aluminum to promote adhesion and a bright finish to the galvanize coating.